Enzymes: Structure and Function

• Enzymes are biological catalysts that accelerate chemical reactions inside living organisms
• Most enzymes are proteins, though some catalytic ribonucleic acid (RNA) molecules known as ribozymes also exist
• Enzymes demonstrate high specificity, catalyzing only particular reactions
• Enzymes are not consumed in the reaction; they can be reused repeatedly

Enzyme Structure

• Enzymes possess a complex 3D structure vital for their function
• The active site refers to the specific region of the enzyme where the substrate binds and where catalysis takes place
• The active site's shape is complementary to the shape of the substrate
• Certain enzymes require cofactors (inorganic ions) or coenzymes (organic molecules) to be active
• Apoenzyme describes the enzyme without its cofactor or coenzyme
• Holoenzyme is the term for the enzyme combined with its necessary cofactor or coenzyme, which makes it catalytically active

Enzyme Function

• Enzymes speed up reactions by decreasing the activation energy
• Activation energy: the energy needed to initiate a reaction
• Enzymes do not alter the equilibrium of a specific reaction; they only accelerate the rate at which equilibrium is attained
• Enzymes create a temporary enzyme-substrate complex
• The substrate attaches to the active site, and the enzyme then catalyzes the reaction
• The product is released following the reaction, and the enzyme is then available to catalyze another reaction

Enzyme Specificity

• Enzyme specificity can differ; some enzymes are extremely specific, whereas others catalyze many similar reactions
• Lock-and-key model: the active site features a rigid shape that precisely matches the substrate
• Induced-fit model: the active site has a flexible shape and alters to better fit the substrate after binding
• Specificity is governed by both the unique amino acid sequence and the structure of the active site

Factors Affecting Enzyme Activity

• Enzyme activity is subject to temperature, pH, substrate concentration, and inhibitors
• Optimal temperature: the temperature wherein the enzyme exhibits maximum activity; activity diminishes above the optimal temperature
• Optimal pH: the pH wherein the enzyme exhibits maximum activity; enzymes are sensitive to pH changes
• Greater substrate concentration raises the rate of reaction until all active sites are saturated; the reaction rate hits a maximum (Vmax)
• Enzyme activity can be regulated by inhibitors

Enzyme Inhibition

• Inhibitors diminish enzyme activity
• Competitive inhibitors attach to the active site, which prevents substrate binding
• Non-competitive inhibitors attach to an alternative site on the enzyme (allosteric site); this changes the shape of the active site, which reduces its activity
• Irreversible inhibitors bind covalently to the enzyme, permanently deactivating it
• Reversible inhibitors bind non-covalently to the enzyme, enabling activity to be restored once the inhibitor is removed

Enzyme Regulation

• Allosteric regulation: the binding of a modulator molecule to an allosteric site (a site other than the active site)
• Feedback inhibition: the product of a metabolic pathway hinders an enzyme earlier in the pathway, thus governing the quantity of product synthesized
• Covalent modification: the addition or removal of chemical groups (such as phosphorylation), which influences enzyme activity
• Proteolytic cleavage: some enzymes are synthesized as inactive precursors (zymogens) and are activated via cleavage of particular peptide bonds

Enzyme Kinetics

• Enzyme kinetics studies the rate of enzyme-catalyzed reactions
• Michaelis-Menten kinetics describes the relationship between substrate concentration and reaction rate
• Michaelis constant (Km): the substrate concentration at which the rate of reaction is half of Vmax; it signifies the enzyme’s affinity for the substrate
• A low Km signifies high affinity, whereas a high Km signifies low affinity
• Lineweaver-Burk plot: a double reciprocal plot of the Michaelis-Menten equation, which is used to determine Km and Vmax
• Turnover number (kcat): the number of substrate molecules that one enzyme molecule can convert per unit of time

Enzyme Classification

• Enzymes are categorized into six primary classes depending on the sort of reaction they catalyze
• Oxidoreductases: catalyze oxidation-reduction reactions
• Transferases: catalyze the transfer of functional groups
• Hydrolases: catalyze hydrolysis reactions (addition of water)
• Lyases: catalyze the breaking of chemical bonds via means other than hydrolysis or oxidation
• Isomerases: catalyze the conversion of one isomer to another
• Ligases: catalyze the joining of two molecules, frequently linked with ATP hydrolysis

Applications of Enzymes

• Enzymes have uses across various industries, including food production, pharmaceuticals, and diagnostics
• In the food industry: amylases are employed to break down starch, and proteases are employed to tenderize meat
• In medicine: diagnostic enzymes that are found in blood tests point to tissue damage or disease
• In pharmaceuticals: enzymes serve as drug targets and are involved in drug synthesis
• In detergents: proteases and lipases are employed to eliminate protein and fat stains
• In biotechnology: restriction enzymes are implemented in DNA manipulation, and DNA polymerases are implemented in PCR

Isozymes

• Isozymes represent different forms of the same enzyme that catalyze the same reaction yet differ in amino acid sequence, regulatory characteristics, and tissue distribution
• They enable precise control of metabolism inside different tissues
• Creatine kinase (CK) represents one example of an enzyme whose isozymes are present in heart, muscle, and brain tissue
• Lactate dehydrogenase (LDH) also possesses a number of isozymes, which can be used diagnostically